#include <stdio.h>

int main(int argc, char *argv[])
{
    FILE *fp;

    fp = fopen("abc.txt", "a");

    fseek(fp, 6, SEEK_END);

    fputc('x', fp);


    if (fclose(fp) == EOF) {
        perror("fclose");
        return EOF;
    }

    return 0;
}
